Atlanta — Four Oakland, Calif., police officers shot down. An Alabama man strolling a small town with a rifle, looking for victims. Seven elderly people shot dead at a North Carolina nursing home. And on Sunday, six people, including four kids, died in an apparent murder-suicide in an upscale neighborhood in Santa Clara, Calif.

The details in all these cases are still emerging. In most, the exact motive has yet to be determined – or may never be fully understood.

On a broader level, however, such incidents may be happening more often because an increasing number of Americans feel desperate pressure from job losses and other economic hardship, criminologists say.

"Most of these mass killings are precipitated by some catastrophic loss, and when the economy goes south, there are simply more of these losses," says Jack Levin, a noted criminologist at Northeastern University in Boston.

Direct correlation between economic cycles and homicides is difficult to prove, cautions Shawn Bushway, a criminologist at the University at Albany in New York. But an economic downturn of this breadth and depth hasn't been seen since data began to be collected after World War II, he also points out. "This is not the average situation," Mr. Bushway says.

Still, criminologists do say that certain kinds of violent crimes have risen during specific economic downturns. The recession in the early 1990s "saw a dramatic increase in workplace violence committed by vengeful ex-workers who decided to come back and get even with their boss and their co-workers through the barrel of an AK-47," Mr. Levin says.

And in the midst of this downturn, one study released Monday in Florida finds a link between domestic violence and economic tragedies like job loss and foreclosures. The Sunshine State saw an almost 40 percent jump in demand for domestic-violence centers, an increase related to the state of the economy, the study says. George Sheldon, secretary of Florida's Department of Children and Families, calls the situation "the worst I've seen in years," according to the Associated Press.

The potential link between murder-suicides and the economy is an area of study for the Violence Policy Center in Washington. "We've been looking at this issue of whether there are more murder-suicides … [and] a pattern is starting to develop that may point in that direction," says Kristen Rand, legislative director at the center. "Between the Texas Tower shootings in the 1960s until the McDonald's massacre in 1984, it was extremely rare to see these types of mass shootings. Now we're seeing them much more often, and they do seem to happen in spurts."

To be sure, the gun-control debate is heating up, especially after the recent Alabama shootings where a man killed 11 people, including himself, using semiautomatic, military-style weapons. Gun-control advocates point to gun proliferation as a major cause for the loss of life, especially when families turn on themselves. That appears to be the case in the Santa Clara shootings.

But the root cause of the violence goes deeper than gun ownership, some argue. "Social isolation is a huge factor" in a country as large and transient as America, which places big emphasis on personal results, Levin says. "If you look at where many of these mass killings have occurred lately, they're in states that have lots of strangers, transients, and drifters, who don't have support systems to get them through tough times," he says.

In the incident in Oakland, which occurred March 21, a parolee shot two officers during a traffic stop, then shot two others during an ensuing manhunt. The parolee also died. It was the biggest single-day, gun-related loss of life for law enforcement in the US since 1993.

The shootings at the nursing home occurred Sunday. Chris McKenzie, police chief in Carthage, N.C., said the gunman, who was killed, may have targeted the home because his estranged wife works there.